1. Field of the Invention
The present invention relates to a conductive member which is used in, for example, a copying machine, a laser-beam printer and a facsimile machine in an electrophotographic system, and disposed adjacent to a subject to be charged such as an image carrier. The present invention also relates to a process cartridge having the conductive member, and an image forming apparatus having the process cartridge.
2. Description of the Related Art
A conductive member is used as a charging member for performing a charging process to an image carrier such as a photoconductor and a transferring member for performing a transferring process to toner on the image carrier in an image forming apparatus of a conventional type in an electrophotographic system, including electrophotographic copying machines, laser beam printers and facsimile machines or the like. The conductive member which is used as a charging member will be explained below.
FIG. 1 is an explanatory diagram of a conventional image forming apparatus in an electrophotographic system.
In FIG. 1, reference number 11 indicates an electrostatic latent image carrier (photoconductor) on which an electrostatic latent image is formed, 12 a charging member (charging roller) which is disposed to contact with or disposed adjacent to the image carrier 11 and performs a charging process, 13 exposure light such as laser light or light reflected from an original, 14 a toner carrier (development roller) which transfers toner 15 onto the electrostatic latent image on an image carrier, 16 a transfer member (transfer roller) which transfers a toner image on the photoconductor onto a recording medium 17, and 18 a cleaning member (blade) which cleans the photoconductor after the transfer process. Moreover, reference number 19 indicates waste toner, which had remained on the photoconductor and then is removed by the cleaning member 18, 210 a development device, and 211 a cleaning device.
The other functional units generally required in the electrophotographic system are omitted in FIG. 1.
In the above image forming apparatus, an image is formed through the following processes as mentioned below:    1. The charging roller charges a surface of the photoconductor at a desired electrical potential.    2. An exposure device irradiates the photoconductor with image light to form an electrostatic latent image corresponding to a desired image on the photoconductor.    3. The development roller develops the electrostatic latent image with the toner to form a toner image on the photoconductor.    4. The transfer roller transfers the toner image on the photoconductor onto a recording medium.    5. The cleaning device cleans the image carrier by removing the toner which is not transferred and remains on the image carrier.    6. The recording medium on which the toner image is transferred by the transfer roller is sent to a fixing device (not shown). The fixing device fixes the toner onto the recording medium by applying heat and pressure to the toner.
The desired image is formed on a recording medium by repeating the above processes 1 to 6.
An image forming apparatus using a contact charging method in which the charging roller is brought into contact with the image carrier has been known as the image forming apparatus using such a general charging method in which the foregoing charging roller is used. Nevertheless, the image forming apparatus using the contact charging method has disadvantages as follows.    (1) A substance constituting the charging roller exudes from a charging roller and is adhered onto a surface of an image carrier to be charged, and then traces of the charging roller remain on the surface of the image carrier.    (2) When an AC voltage is applied to the charging roller, the charging roller being in contact with the image carrier vibrates. This causes charging noise sounds.    (3) Since the toner on the image carrier is adhered on the charging roller, in particular, the toner is easy to be adhered due to the above exuded substance, a charging performance of the charging roller is degraded.    (4) A substance constituting the charging roller is adhered to the image carrier.    (5) In a case where the image carrier remains out of operation for a long period of time, a permanent deformation takes place in the charging roller.
An image forming apparatus using an adjacent charging method in which the charging roller is disposed close to the photoconductor has been disclosed as techniques for solving the foregoing problems in each of Japanese Patent Application Publication Numbers Hei 3-240076, 2001-312121, and 2005-91818. In this method, the charging roller is disposed to face the photoconductor with a gap which is 50 to 300 μm as a distance between the charging roller and the photoconductor at a closest approach point, and the photoconductor is charged by applying a voltage to the charging roller. In the case of the adjacent charging method, since the charging roller is not in contact with the image carrier, the image forming apparatuses using this adjacent charging method are free from the problems occurring in image forming apparatuses using the conventional contact charging method, such as the problem of “the adherence of the substance constituting the charging roller to the image carrier” and the problem of “the permanent deformation which takes place in the charging roller in the case where the image carrier remains out of operation for a long period of time”. In addition, the image forming apparatuses using the adjacent charging method are less likely to “degrade the charging performance of the charging roller due to the adherence of parts of the toner on the image carrier to the charging roller” than the image forming apparatuses using the contact charging method.
Characteristic properties required for the charging roller used in the adjacent charging method are different from those required for the charging roller used in the contact charging method. Generally the charging roller used in the contact charging method is formed by coating an elastic member such as a vulcanized rubber or the like around a core shaft. In order to charge uniformly the image carrier using such a charging roller, it is required that the charging roller should be in contact uniformly with the image carrier.
However, in a case where the charging roller formed from an elastic member such as a vulcanized rubber or the like is used in the adjacent charging method, there are such problems as listed in the following:    (1) Although it is necessary to dispose gap maintaining members such as spacers or the like at both ends of the charging roller corresponding to none image areas in order to provide a gap between the charging roller and the image carrier, it is difficult for the gap to be kept uniformly because of the deformation of the charging roller formed from the elastic member. As a result, this causes potential variation and image unevenness resulting from the potential variation.    (2) It is easy for the vulcanized rubber constituting the elastic member to become strained and deform with time, and as a result the gap varies with time.
To solve such problems it has been proposed to use a non-elastic member made of a thermoplastic resin which makes it possible to make uniform the gap between the image carrier and the charging roller.
It is known that a charging mechanism in which the surface of the image carrier (photoreceptor drum) is charged through the charging roller follows the Paschen rule within a small space between the charging roller and the image carrier in discharging. In order to keep the image carrier at a predetermined charge potential level, it is necessary to control the electrical resistance value of the thermoplastic resin within a semi conductive range of about 106 to 109 Ωcm.
Among methods to control the electrical resistance value of the electrical resistance adjusting layer, there is one to disperse conductive materials such as carbon blacks or the like in the thermoplastic resin. However, such a method will cause larger variation of the electrical resistance values, resulting in partial charging failure which leads to a problem of image forming failure.
Another method to control the electrical resistance value of the electrical resistance adjusting layer is to add an ion conductive material. Since such an ion conductive material may be dispersed at a molecular level in a matrix resin, the irregular variation of the electrical resistance values is smaller than that dispersed with the conductive pigments, resulting in smaller partial charging failure which will not affect the image quality. However, the ion conductive material which has a low molecular weight has a character to easily bleed out to the surface of the matrix resin. When the ion conductive material bleeds out to the surface of the charging roller, toner is adhered and fixed, leading to a problem of failure in an image formation.
In order to prevent the bleeding out of the ion conductive material, it has been proposed to use a high molecular weight ion conductive material which is dispersed and fixed in the matrix resin. In such a case, it is difficult for the high molecular weight ion conductive material to bleed out to the surface of the matrix resin. Japan Patent Number 3180861 discloses a polymeric ion conductive material having a quaternary ammonium group, which has a lesser bleeding out property with time. However, in the case of the polymeric ion conductive material, since the resistance value depends strongly on an environmental condition such as temperature and humidity, there are problems of abnormal discharge due to a low resistance value or charging failure due to a high resistance value depending on an additive rate or a condition such as temperature and humidity. In particular, image forming failure due to the abnormal discharge can easily occur under a condition of low temperature and low humidity (LL).
Furthermore, such a polymeric conductive material dispersion system in which a polymeric ion conductive material is dispersed to form a sea-island structure is disclosed, for example, in Japan Patent Application Publication Number 2005-92134. In a case where island portions of the sea-island structure are made of the polymeric ion conductive material, since a current is prevented by an insulating matrix resin, there are problems in that the resistance value of the electrical resistance adjusting layer is not reduced within the semi conductive range, or that the resistance value depends more strongly on voltage. Additionally, there are problems in that strength at a weld portion formed in molding is degraded if a size of each of the island portions is large, and that the resistance value varies. Due to this degradation of the strength, if a resin which has a low mechanical strength or resins which are not compatible with each other are used for a matrix resin, cracks at a weld portion of the electrical resistance adjusting layer, which is for example, a weld line formed in molding of the electrical resistance adjusting layer, occur due to electrical or mechanical stresses when used or volume fluctuation with time or environmental change. In addition, variation of resistance values at the weld portion, that is to say, partial unevenness of the resistance values may cause partial image failure in some cases.